The present invention relates an image area discriminating device for automatically discriminating whether each of the scanned areas of an input image has been previously subjected to halftone dotting treatment or not. Such an image area discriminating device is assembled in a digital copying apparatus or like equipments.
For example, in a copying apparatus, images on an original document are read by a CCD (charge coupled device) image sensor or like while dividing each image into a plurality of very small areas, i.e. plural picture elements, so that these picture elements are detected by the image sensor to generate analog electric signals which are outputted as the outputs from the image sensor and then processed through an A/D (analog/digital) converter to be converted into digital signals. The digital signals are subjected to various processing sequence and fed to a recording apparatus to form copied images. In a recording apparatus of this type, it is a common practice to effect binary recording for the recording/unrecording, since it is difficult to change the density level for each of the plural picture elements. However, in case where halftone images are contained in the original document, it is necessary to reproduce such halftone images. It has been already proposed to reproduce halftone images in such a binary recording apparatus by means of a dither method, density pattern method or sub-matrix method. Halftone images may be reproduced by using any one of these known methods. On the other hand, when an original document is subjected to halftone dotting treatment, a relatively fair copy can be obtained as far as the densities of the images are varied gently as is the case of copying a photographic image, but the quality of the copy is seriously deteriorated in that the profiles of the copied images become blurred to make it difficult to read the copied letters or the stains on the background of the document sheet are reproduced in the copy if the densities of the images on the document are changed in the binary fashion as is the case of copying letters.
A fair copy of images on a document may be obtained by simple binary processing rather than subjecting them to halftone dotting treatment when the images are letters or like images. A preferable copying mode may be selected by an operator depending on the characteristics of the original document by the provision of a switch for instructing whether the document should be subjected to halftone dotting treatment or not. However, there are many documents which contain halftone images such as photographs and binary images such as letters scattered randomly, as is the case with pamphlets. In such a case, the quality of copied photograph is deteriorated if the binary processing mode is selected, whereas the quality of copied letters is deteriorated if the halftone processing mode is selected.
The digital copying apparatus of this type has another disadvantage. When the image is read by dividing the same into small picture elements which are detected by a line sensor, there is a risk that the problem of a moire might appear in the recorded image caused by the interference between the pitch of periodic change in density of the image on the document and the pitch of the arranged image detecting sensors (sampling pitch), if the change in density of the image on the document is periodic. For instance, if the document bears a halftone dotted printing image, the change in density of the image is periodic As a result, the moire problem appears by the interference between the pitch of the periodic density change and the sampling pitch of the detecting sensor. For example, when the rosolving power of the image detecting sensor is 16 picture elements/mm, the moire problem tends to appear in the detection signals if the density of the halftone dotted printing is in the range of from 133 lines (about 10.5 picture elements/mm) to 200 lines (about 16 picture elements/mm), the range being approximate to the resolving power of the sensor. The Moire problem might appear when the density of the halftone dots is out of the aforementioned range. The moire problem appears more frequently if the density of the halftone dots is within a range to cause marked variations in width of the signal.
The halftone dotted printing per se is a kind of pseudo-halftone reproduction, and the density change for each picture element is binary which may be coded as a 1/0 or recording/unrecording. In the halftone dotted printing, the average density of the image is stepwisely changed by changing the pitch of the dots or by changing the dimensions of the dots, wherebY a copy having a general appearance of halftone density is reproduced. Accordingly, setting the problem of moire aside, halftone dotted images on a halftone dotted printing document can be reproduced as recorded images to obtain a fair copy by processing the signals in the binary processing mode. However, in practical copying of images on a document, the quality of a copy is seriously deteriorated due to the aforementioned moire problem when the images have preveiously been subjected to halftone dotting treatment and have densities of a particular range.
On the other hand, when the signals obtained by detecting the images are subjected to halftone dotting treatment to convert then into binary signals, the densities of the plural picture elements are averaged or the threshold level is varied in the course of processing, whereby appearance of moire in the copied image is prevented or the adverse influence caused by the moire problem is suppressed. Although the densities of the copied images are reproduced in pseudo-halftone, the halftone dots on the copy are not those directly reproduced from the halftone dots on the original document but are formed by the halftone processing particular to the copying apparatus employed. Accordingly, if the images on the original document are those which are formed by halftone dotted printing or by a digital copier, it is recommended to select the copying mode for subjecting the images to halftone processing although the picture elements forming each image are recorded by binary-coding